Signings is obviously a legitimate word, but it's usually used when there's an act of signing involved, like with book signings or hearing-impaired signing. I suppose one might consider a road sign to be an "act of signing/signaling" but it seems like a stretch.

Is this a common use of the word I've just missed out on? Construction jargon? Or just a construction worker's peculiar choice?

@tchrist - Driving on a highway that was under construction. The full text was something along the lines of "Delays ahead. Follow alt route signings", presumably suggesting to take the faster alternate route to avoid the delays.
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LynnAug 16 '12 at 15:25

@FumbleFingers Not even that in the UK: "Diversion Follow ◙" and just the symbol on subsequent signs. The OP's is probably a one-off and maybe the sign-maker got the spacing wrong: found that "sign" was in the wrong place and filled in with "ing" and then a final "s". Or, just an odd choice. This is almost Too Localised.
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Andrew Leach♦Aug 16 '12 at 15:42

(a) There must be on site, at all times when the works are in progress and workers are on site, at least one person who has been issued with a valid construction skills registration card relating to either “health and safety at roadworks” or “signing, lighting and guarding”. Furthermore the works must be supervised by a competent person who has been issued with a valid construction skills registration card relating to signing, lighting and guarding on roads.

In other words, as the OP suspected, this is very likely jargon particular to the construction industry. One explanation for the use of signings could be due to their temporary nature (during construction). I couldn't make out if these regulations also involved a person manually directing traffic (often seen in some countries), a requirement which would make the use of signing more obvious.

I also have no idea if the occurrence in the US is a one-off or not. But I did come across one reference which might suggest that it is possible.